The FSK modulated signal is signal carrying digital information to be transmitted through a frequency modulation, and therefore if there is a frequency error in the modulated signal received in a receiver, bias distortion can be introduced in a demodulated signal. Therefore, in order to avoid this, an error of the center level of the demodulated baseband signal, that is a DC offset component which has been introduced by the frequency error, needs to be detected and the frequency error to through an AFC (Automatic Frequency Control) so that the detected DC offset component is made zero, or a correction must be carried out so that this error is cleared away by using the baseband signal. Whit regard to either method, in the case where a packet is short in length, bit synchronization and frame synchronization must be established as soon as the signal was received, and for this purpose the effect of this frequency error must be corrected, prior to these synchronizing operations. In order to carry out the bit synchronization, there are in general two methods, one using squaring/band-pass filtering, the other directly driving a DPLL (Digital Phase Locked Loop) with the baseband signal. In the former method, a DC offset can become irrelevant, but it needs a relatively long processing time, which means that this method is not suitable for a faster operation. Therefore, the latter method is utilized usually for this purpose. Since it must use a center level detecting system, various such systems have been developed in the prior art. Typical examples of the prior art center level detecting systems which are used in connection with a demodulated baseband signal are as follows.
In a first center level detecting system, positive and negative peak values of the bit synchronization signal are detected, and a center level error, that is a DC offset component, is found from the mean value of these detected positive and negative peak values. In a second center level detecting system, a new synchronization signal component is formed from the original synchronization signal which includes a DC offset, the DC component of said newly formed synchronization signal component having been removed, said formed synchronization signal component having the same amplitude as that of the original synchronization signal and its phase reversed to that of the original synchronization signal, and the newly formed synchronization signal component is added to the original signal to cancel the synchronization signal component, so as thereby to pick up only the DC offset component which has been included in the original signal. In a third center level detecting system, a DC offset component is detected by comparing the demodulated baseband signal with its rectangularly shaped signal having the same amplitude. In the first and second systems it is not necessary to use a smoothing circuit, but the third system does require it, and therefore the first two systems are faster than the third system. Since transient phenomenon is produced in an AC coupling circuit and a phase shifting circuit of the second system, the first system is faster than the second system. Therefore, the first center level detecting system can be said to be the fastest system of the three.